1. Field of the Invention
This invention relates to the field of photo-spectrometers, and in particular, to photo-spectrometers realized in a standard semiconductor process, to an IC compatible photo diode spectral selectivity/enhancement method using multiple pn junctions and to a method for determining light spectra using optical detectors with controllable spectral response.
2. Description of Related Art
Photo-spectrometers are used in a variety of scientific and industrial applications including atmospheric measurements, medical instruments, flame monitoring in commercial hydrocarbon-fueled furnaces and aircraft engines, and a number of colorimetry applications ranging from color scanners to industrial process control. Even red-green-blue color imaging is a limited application of photo-spectroscopy.
Many photo-spectrometers consist of an array of photodiodes, an optical filtering or dispersive element, collimating and focusing optics, and data collection electronics. Each technique allocates a wavelength band to one photodiode in the array. Although these systems are capable of resolution better than 1 nm, they require precise alignment, are relatively expensive, and are too large for micro-instrumentation applications.
Micro-fabrication techniques are used to produce photo-spectrometers that overcome the disadvantages of the systems described above. There is on-going research to produce spectrometers based on silicon-on-insulator (SOI) photodiode arrays. Fabry-Perot micro-spectrometers have been produced using bulk and surface micro-machining techniques, and a spectrometer composed of a micro-fabricated diffraction grating attached to a CCD array has been realized. In addition, a calorimeter realized using an amorphous silicon (Si) Schottky photodiode has also been reported.
These micro-fabricated spectrometers or calorimeters are capable of good spectral resolution and are appropriate for micro-instrumentation. However, these devices require fabrication steps or materials that are not part of a low-cost, standard IC process and do not take full advantage of mass production in this mature technology. None of these spectrometers can be realized in a standard semiconductor process using only the standard masks, materials, and fabrication steps. Such prior art devices cannot be integrated with additional analog, digital, and wireless circuits constructed in the same semiconductor process to produce a true instrument-on-a-chip.